Over the last 400 yrs or so the simple ‘telescope has come along way,  and today there is a bewildering array on the market. Two basic designs though have been used for hundreds of years:- the refractor and the reflector, a third type, the Catiadoptic or Cat, is a more recent innovation and is fast becoming as popular as the others. Here are some basic facts, advice and tips on what to look for in choosing a telescope for a novice or more experienced observer.                                                     

Refracting telescopes employ lenses at either end of a sealed tube, light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube. If looked after little or no maintenance is required. The objective lens is permanently mounted; sealed and aligned therefore dirt and air currents should not enter. Refractors are excellent for lunar, planetary and binary star observing especially in larger apertures, they can also be utilised for terrestrial viewing unlike reflecting scopes. More expensive refractors employ apochromatic, fluorite, and ed lens designs, the resultant image being as good as it gets. All other refractors use achromatic lens designs – perfectly satisfactory for most observers.

A reflecting telescope employs mirrors to collect and direct the light and an eyepiece to magnify the image. Reflectors are solely used for astronomical purposes, the image being upside down and back to front, a detail that matters little when viewing celestial bodies. Reflectors are ‘open tube’ instruments and over time the mirrors are subject to deterioration, periodically requiring re-aluminizing. Dependent on mirror size this can cost anything from £60 - £120 – a hidden cost to keep in mind. The mirrors may also need to be re aligned or ‘collimated’ from time to time otherwise the image becomes distorted. Sizes for aperture size, reflectors are the cheapest telescope type, particularly the ‘dobsonian’ mounted design. Apertures under 200mm are reasonably compact and portable up to focal lengths of 1000mm. Reflectors are best for observing deep sky objects such as galaxies, nebulae and star clusters but are still good for lunar and planetary observing.                                                                                                                                                                                                                                                                                                                                                                                                              Catadioptrics use a combination of mirrors and lenses to fold the optical light path, before forming the image. Cat’s are therefore compact and portable, but are heavy for apertures exceeding 250mm in diameter. Cat’s are excellent all round scopes that require little maintenance and can be used for terrestrial viewing. The downside is that they are more expensive than reflectors of equal aperture and offer less colour contrast compared to refractors and reflectors. Without dew shields they are also susceptible to dewing up, a problem in the UK. The most common design is Schmidt-Cassegrain. Many Cat’s have GOTO capability that allows the scope to point to any object in its computer database. Personally if you are a novice observer I would avoid. Irrespective of design all scopes should come with a finder scope of some type, which goto if aligned carefully allow the targeting of objects a synch. Allow time to do this, believe me it is well worth the trouble

A scope with superb optics is useless if the stand and mount it comes on are unsteady or unworkable. Mounts come in two basic designs, Alt azimuth and Equatorial.  Alt azimuth mounts are simple in design, moving in two directions - up and down and side to side, normally on top a tripod. Equatorial mounts are heavier, more complicated, but have advantages in that the arrangement allows the scope to move parallel with the Earth’s rotational axis, thus reducing the need for constant adjustment of fine motion controls. If a motor drive is fitted to the axes, celestial objects can be tracked without any adjustment.

Most telescopes are now supplied with 32mm (1 ¼ inch) barrel size eyepieces, with the focal length stamped on the side, eg 25mm or 10mm etc. To calculate the magnification divide this number into the focal length of the scope, so a telescope with a focal length of 1000mm with a 25mm eyepiece will yield a magnification of 50. Remember the following golden rule of magnification, the upper limit to apply is 50 per 25mm (1 inch) of aperture.                                                         

Before buying a telescope ask yourself a few questions. What is my budget? What do I want to observe? Is portability a concern? Can I be bothered with a complex looking mount. A beginners or childs’ 60-70 mm refractor can be purchased for as little as £75, an entry level reflector for £130.  For better quality 80 - 102mm refractors expect to pay between £170 and £300. A plethora of more robust 110mm (4 ½ ) reflecting scopes can be purchased for £180-£280. The most cost effective type of astronomical scope is the Dobsonian reflector - a reflecting scope mounted on an alt azimuth box mount, these can be picked up for as little as £150 for a 150mm aperture – an excellent buy. Cat’s (90mm) start at £260, but expect to pay £1000 plus for anything over 200mm. 

Take a look through the pages of an astronomical magazine such as ‘Astronomy Now,’ there you will find pages of ads for scopes of every type and cost. If you are still unsure the local astronomical society will point you in the right direction.

                     Remember a scope is not just for Christmas!.